For example, JP-A-2008-144784 discloses an example of the annular seal member. The annular seal member (packing) disclosed in JP-A-2008-144784 has an annular base part and a lip part that axially extends from one end of the base part. Also, the annular seal member has a pressure guidance path on an outer periphery of the base part, which guides fluid pressure in a space (a space of a side for which it is required to avoid pressure accumulation) partitioned by a seal part toward a contact part of the lip part with respect to a bottom face of a seal recess.
The annular seal member is accommodated in an annular seal recess and interposes the base part between a shaft and a shaft hole by compressing the base part in an axial direction. A seal edge in vicinity of a free end of the lip part is brought into close contact with a seal mating face (bottom face of the seal recess).
According to the annular seal member disclosed in JP-A-2008-144784, when a fitting interference is excessively increased due to non-uniformity in size and the like (when a diameter of the bottom face of the seal recess is smaller than a diameter of a base end of the lip part), a slit, which is provided as the pressure guidance path, is deformed in the vicinity of the base end of the lip part due to elastic deformation or creep deformation, so that the pressure guidance path is obstructed. Thereby, a force of separating the lip part from the seal mating face (a pressure difference of partitioned spaces A and B, and this is referred to as a valve opening pressure) is to be varied, so that it is unlikely possible to obtain a stable pressure release function.
Also, when many pressure guidance paths are provided or a width of the pressure guidance path (slit) is increased so as to avoid the obstruction of the pressure guidance path, it is not possible to secure a straining force (surface pressure) that uniformly and sufficiently presses an inner diameter side of the annular seal member to an outer periphery of the shaft. As a result, the reliability of the seal is deteriorated, so that a possibility of liquid leakage is increased.